In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as better control of contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
Cone beam helical scanning also is known. A cone beam scan is performed using a multi-dimensional detector array instead of a linear detector array as is used in a fan beam scan. In a cone beam helical scan, the x-ray source and the multi-dimensional detector array are rotated with a gantry within the imaging plane as the patient is moved in the z-axis synchronously with the rotation of the gantry. Such a system generates a multi-dimensional helix of projection data. As compared to fan beam helical scanning, cone beam helical scanning provides improved slice profiles, greater partial volume artifact reduction, and faster patient exam speed.
Generally, in cone beam helical scanning, approximately one-helical-pitch worth of data on each side of the image slice is used to generate the image data. Specifically, image data on each side of the image slice, and 360.degree. apart is interpolated to reconstruct an image slice. The method for generating the image using one-helical-pitch worth of data on each side of the slice is sometimes referred to as a "360.degree. interpolation" method, and is effective in reducing inconsistency artifacts.
The one-helical-pitch interpolation method described above generally requires a total of two-helical-pitch (720.degree.)worth of data to reconstruct each slice, i.e. , one-helical-pitch worth of data on each side of the slice. The 360.degree. interpolation method, therefore, does not provide satisfactory results at end regions along the z-axis. Specifically, any slice to be reconstructed within the "first" or "last" helical pitch worth of data along the z-axis does not have the requisite two-helical-pitch worth of data. These end regions are sometimes referred to herein as "dead regions." Moreover, using a long range interpolation to reconstruct slices broadens the slice profile.
Also, in some applications, reduced scanning time or increased volume coverage is required. Without changing other system parameters, the time reduction and coverage increase can be achieved by moving the table faster, which results in each slice being supported by less than 27r worth of cone beam data.
It would be desirable to provide, in a cone beam helical image reconstruction, a manner for reducing the extent of the dead regions and slice profile broadening. It also would be desirable to enable the table speed to be increased in a cone beam system.